CHROMOSOME ABERRATIONS IN ANIMALS 645 



2-2c. Deficiencies and Duplications. A segment, rather than an entire 

 chromosome, may be eliminated from the normal complex or added to it 

 as a result of irradiation damage. The loss of a section of a chromosome 

 is designated as a "deficiency," and the repetition of a section as a 

 "duplication." The length of the section removed or added and the 

 genetic properties of the chromatin involved determine the extent of 

 genie unbalance and the capacity of the cells receiving the altered 

 chromosomal complement to survive. Deficiencies and duplications con- 

 fined to heterochromatic ("inert") regions, such as exist in the proximal 

 third of the X chromosome and throughout the Y chromosome of D. 

 melanog •aster, have less effect in modifying normal developmental proc- 

 esses than alterations of comparable length within the euchromatic 

 ("genetically active") portions of the chromosomes. (As indicated 

 previously, the loss of the entire Y leads to the production of an XO male, 

 which is viable though sterile; and the addition of a Y, as in an XXY 

 female, does not appreciably reduce viability or fertility.) 



Deficiencies confined to euchromatic regions have, in general, more 

 deleterious effects than duplications of comparable length. Studies on 

 Drosophila have indicated that deficiencies are usually lethal when 

 homozygous or hemizygous (as, for example, when a deficient X chromo- 

 some is present in duplicate in an XX female zygote, or as a single X in an 

 XY male zygote), although some short terminal deficiencies are not 

 (Demerec and Hoover, 1936). Many of the so-called "lethal mutations" 

 of Drosophila are attributable to minute deletions (Slizynski, 1938). A 

 deficiency may at times be detected by the absence of specific marking 

 genes. Thus, a deficiency induced in the 1A5-8 region of the X chromo- 

 some of D. melanogaster by irradiation of wild-type males, which were 

 mated with y sc females, was detected by the appearance of female 

 progeny with yellow body color (Sutton, 1943). A deficiency for band 

 3C7 of the X chromosome acts as a dominant (Notch), producing flies 

 with serrated or notched wings (Demerec, in Demerec and Kaufmann, 

 1937). Such correlations of phenotypic effects with chromosomal aber- 

 rations have been useful in determining the loci of specific genes on the 

 chromosomes, as is illustrated in Fig. 9-10. 



Deficiencies may be either terminal or intercalary in position. Ter- 

 minal deficiencies are frequently detected in irradiated cells as fragments 

 separated from the portion of the chromosome having the centromere or 

 spindle-attachment region (Figs. 9-3 and 9-4). A terminal fragment 

 detached from an ordinary, or monocentric, chromosome lacks the 

 centromere essential for its normal transportation to the spindle pole at 

 anaphase. Consequently, it is usually not included in either of the 

 daughter nuclei (however, see Carlson, 1938b). Fragments detached 

 from chromosomes having compound or diffuse centromeres — such as 

 those of Ascaris, the coccids, or the bearberry aphid — provide an excep- 



